I am not sure is we are discussing the philosophy of women in general, or if we are discussing this one phrase. I mean, no one is born a woman. A woman is an adult female. I am not sure if she was being literal, in the sense that the life experience of a female is what makes her a woman, or if she was referring to her own feminist views.
Because the social roles of women are different around the world, depending on the culture, it's easy to see her argument. On the other hand, the natural instinct of a woman to have an care for children can not be denied by anyone that has witnessed it. Just as children we were amazed to watch the mother cat care for her kittens or dog care for her puppies, human females are no different in this regard. I remember wondering how that cat that we got as a kitten, knew what to do? It certainly was not learned behavior.
So, perhaps in the context of her own political and philosophical views, one is not born a woman. However, as far as the instinctual part, well that is prewired in the brain. Just as a woman is born with all the eggs she will ever have, she is also born with a certain part of her womanhood.
I guess the only mind experiment we might perform is to ask, are there females that do not become women? It's kinda like saying, no insects are born butterflies, they become butterflies. Indeed, but they are also all predetermined to do so.
Love always,
Elizabeth